1. Field of the Invention
The present invention relates to a light emitting device in which a light emitting element and an element for supplying current to the light emitting element are provided for each of a plurality of pixels and to a method for fabricating the light emitting device.
2. Description of the Related Art
A light emitting element has high visibility since it emits light by itself, and is most suitable for thinning since it does not need a backlight, which is required in a liquid crystal display device (LCD); further, there is no limitation of viewing angle either. Therefore, a light emitting device with the use of a light emitting element has been attracting attention as a display device which replaces a CRT and an LCD. In late years, a light emitting device has been put to practical use; for example, it is provided in a cellular phone or an electronic device such as a digital still camera.
A light emitting element has an anode, a cathode, and an electroluminescent layer sandwiched between the two electrodes. One of the two electrodes is referred to as a pixel electrode hereinafter. In the pixel electrode, the potential is controlled corresponding to a video signal and a pixel electrode in a pixel is separated from pixel electrodes in other pixels. The other electrode in which a common potential is given (referred to as a counter electrode) is normally formed all over to be shared by all pixels, or formed so that pixels of each of RGB have a common counter electrode because it is not realistic to divide the counter electrode as the pixel electrode. Supply of potential to the counter electrode is performed via a connection terminal provided at an end portion of a panel. Specifically, the contact of the counter electrode with a wiring for leading (referred to as a leader wiring) is made, and the counter electrode and the connection terminal are electrically connected by the leader wiring. Then, the contact is provided in a region other than an area where the electroluminescent layer of a pixel area is formed.
As a screen is made larger, the area of the pixel area becomes larger, and potential drop due to the resistance of the counter electrode tends to be significant. Brightness gradient could be observed when viewed as a whole pixel area since the absolute value of voltage Vel applied between electrodes of a light emitting element is reduced in a pixel in which the potential drop of a counter electrode is significant. In order to avoid the above problem, a technology is proposed in Reference 1 (Reference 1: Japanese Patent Laid-Open No. 2002-033198). In the technology, an electrode for an auxiliary (an auxiliary electrode) is formed from a material with low resistance so that the auxiliary electrode is connected to a counter electrode, thereby uniforming the potential in the plane of the counter electrode after the formation of the light emitting element.
As mentioned above, in the cases where the resistivity of a material forming a counter electrode is high, or where the resistance of a counter electrode becomes high due to the increase in the area of a pixel area, it is a very effective measure to form an auxiliary electrode for uniforming the potential in the plane of the counter electrode. However, it is necessary to lay out the auxiliary electrode so as to obstruct as less light from a light emitting element as possible in the case of a type of a light emitting device (a top emission type) in which light emitted from an electroluminescent layer is released from a light transmitting counter electrode. However, even if an auxiliary electrode is formed only in an area which is not overlapped with a pixel area, it is difficult to obtain desirable effect of the uniform potential in the plane of the counter electrode. Therefore, an auxiliary electrode is formed over a counter electrode in the area where light emission is not actually obtained, such as an area between light emitting elements of adjacent pixels.
However, as the size of a pixel is reduced due to developments in higher precision of pixels as well as larger screens, the width between light emitting elements of adjacent pixels becomes less than 20 μm. Accordingly, an auxiliary electrode is required to fit within the above width, and it is becoming difficult to form an auxiliary electrode over a counter electrode by vapor deposition with the use of a metal mask which cannot form a very precise pattern. When an auxiliary electrode is formed over a counter electrode by photolithography, the pattern can be formed with precision of μm or less. However, it is unfavorable since degradation of a light emitting element due to light or moisture might be accelerated in serial steps including exposure, development, and removal of a photoresist. Further, although an auxiliary electrode can be formed by a printing method typified by ink-jet printing, it is undesirable since the number of steps for forming the auxiliary electrode would increase. This problem is the same in vapor deposition and lithography.